Have you got priority in this situation before the vehicle going ahead across the junction?

A flashing yellow signal on the traffic light means that the signals are inactive and general priority rules or road signs apply. In this situation, when performing a left turn, you are obliged to yield to a vehicle approaching from the opposite direction and going straight. This stems from the principle set out in Art. 25 para. 1 of the Road Traffic Act, which requires yielding to vehicles coming from the opposite direction going straight or turning right.